Objective: To examine changes in depressive symptoms as well as between- and within-person associations between depressive symptoms and Type 1 diabetes (T1D) management across the transition from late adolescence to emerging adulthood. Method: Beginning in the senior year of high school, 197 late adolescents with T1D (Mage = 17.77) reported on their student status and living situation, and completed self-report measures of depressive symptoms and adherence to the diabetes regimen, annually at 3 time points. Glycemic control was gathered from hemoglobin A1c (HbA1c) assay kits at the same time points. Results: Results of multilevel models demonstrated high depressive symptoms at baseline, with significant increases in depressive symptoms across time when participants were not living in their parental home, but no change when living with parents. Participants with higher mean levels of depressive symptoms relative to peers (between-person association) had poorer adherence and glycemic control (i.e., higher HbA1c) on average. Within-person fluctuations in depressive symptoms were significantly associated with adherence: greater increases in depressive symptoms (relative to adolescents’ own average) were associated with greater deteriorations in adherence. There was not a significant within-person effect of depressive symptoms on glycemic control. Conclusions: The transition from late adolescence to emerging adulthood is particularly challenging for those with T1D. The findings that individuals with greater depressive symptoms have poorer adherence and glycemic control relative to those with lower depressive symptoms, and that increases in depressive symptoms are associated with declines in adherence, highlight the importance of screening and monitoring depressive symptoms during this life transition.